1. Field of the Invention
The present invention relates to a filtration membrane for oleophilic organic liquids, capable of removing fine impurities that exist in an oleophilic organic liquid from the liquid as efficiently as possible, to a method for producing the filtration membrane, and to a method of using the filtration membrane for filtering an oleophilic organic liquid.
2. Description of the Related Art
Engine oil for cars is needed to have the function for lubrication, cooling, sealing, cleaning, rust inhibition, etc. Therefore, various additives shall be incorporated in it. These additives are deteriorated by various impurities to be introduced thereinto and by high temperatures during driving of cars, and undergo various chemical modifications caused by such impurities. As a result, their performance is degraded. When metals are processed by discharge erosion in oil, the oil is contaminated by impurities such as metal powders.
Contaminated oil that contains such various impurities has been recycled through a filtration membrane. For example, a polyacrylonitrile membrane, a polysulfone membrane or a cellulose acetate membrane as shown in Japanese Patent Publication No. 62-11883 have heretofore been used as the filtration membrane of this type in a method for filtering contaminated oil through a filtration membrane.
However, the conventional filtration membranes involve the following problems.
First, the filtration membranes, as being hydrophilic resins, shall be produced in hydrated conditions. Therefore, if the membranes are used for filtering oleophilic organic liquids, such as oil, they must be converted into oleophilic membranes, prior to use, through complicated solvent substitution where water existing in the membranes is replaced first with a hydrophilic solvent and thereafter with a solvent with stronger oleophilicity in due order thereby finally converting the membranes into oil-containing membranes.
The conversion of hydrophilic filtration membranes into oil-containing membranes shall change the size of the pores in the membranes, and it is impossible to control the size of the pores necessary for filtration. Even if such hydrophilic filtration membranes could be converted into oil-containing membranes, their filtration resistance to oil is large and their filtration efficiency is low since the membranes themselves are made of hydrophilic materials.